Nowadays, more than one GNSS system is available. A GNSS receiver has to process diverse satellite signals modulated by various modulation schemes, especially BPSK and BOC modulation schemes, in the near future.
GPS is the U.S. navigation satellite system, which is a network of satellites continuously transmits high-frequency radio signals. The signals carry time and distance information that is receivable by a GPS receiver, so that a user can pinpoint the position thereof on the earth. Galileo, the emerging European satellite navigation system, offers higher signal power and more robust modulation that will enable users to receive weak signals even in difficult environments. When combined, which provide so called “super constellation”, Galileo and GPS will offer twice the number of satellite sources as currently available. This provides many advantages such as redundancy as well as greater availability for the user. In addition, positioning accuracy and integrity can be improved. Multiple satellite signals of different carriers can be used to resolve carrier phase ambiguity. Since a lot of frequency bands are available, a clean frequency band can be selected from various bands to use for the sake of anti-jamming. Further RAIM (Receiver Autonomous Integrity Monitoring) can also be improved.
GPS and Galileo systems share some signal bands with different modulation schemes (e.g. BPSK and BOC). In the coming new generation GPS, there are also different signals using the same band with different modulation schemes (BPSK and BOC). Table 1 and Table 2 respectively show GPS signal characteristics and Galileo signal characteristics.
TABLE 1SignalCarrier (MHz)ModulationChip Rate (Mcps)L1 C/A1575.42BPSK-R(1)1.023L1 P(Y)1575.42BPSK-R(10)10.23L2 P(Y)1227.6BPSK-R(10)10.23L2C1227.6BPSK-R(1)1.023L51176.45BPSK-R(10)10.23L1M1575.42BOC (10, 5)5.115L2M1227.6BOC (10, 5)5.115L1C1575.42BOC (1, 1)1.023
TABLE 2Carrier Chip Rate BandSignal (MHz)Modulation (MHz)E5E5a data1191.795AltBOC(15, 10)10.23E5a pilot E5b dataE5b pilotE6E6P1278BOCc(10, 5)5.115E6C dataBPSK-R(5)5.115E6C pilotBPSK-R(5)5.115E2-L1-E1L1P1575.42BOCc(15, 2.5)2.5575L1F dataBOC(1, 1)1.023L1F pilotBOC(1, 1)1.023
As can be seen, for example, signals L1 C/A and L1 C of the same band in GPS system have the same carrier frequency and chip rate but are modulated with different modulations, that is, BPSK-R(1) and BOC(1,1). Alternatively, signals L1 C/A in GPS and signal L1F in Galileo also have the same the carrier frequency and chip rate but are modulated with different modulation schemes, that is, BPSK-R(1) and BOC(1,1), respectively. If a receiver is required to process the different signals, the receiver must be provided with different groups of correlation units. For example, a GNSS receiver must have a correlation unit group for processing a BPSK modulated signal and another correlation unit group for processing a BOC modulated signal. When processing the BPSK modulated signal, the correlation unit group for processing the BOC modulated signal is idle. Such a waste is undesirable. Therefore, there is a need for a more efficient and flexible solution.